This invention relates to impact absorbers.
One application for impact absorbers is to support the bumpers of a road vehicle from the body of the vehicle. The impact absorber is so arranged that if the bumper of the vehicle sustains an impact, it can yield against a resisting force provided by the impact absorber. In this way some, at least, of the impact energy is absorbed by the impact absorber, and damage to the vehicle is minimized, or even prevented entirely at reasonably low impact speeds.
An impact absorber has already been proposed which comprises a cylinder, a piston slidable within the cylinder, a valve member arranged to cooperate with a seat in a valve housing to control fluid flow out of the cylinder working chamber on one side of the piston, and means biasing the valve member on to the seat. The two sides of the piston are connected by passages through the piston which are normally closed by an annular disc biased against the piston by its own resilience or by one or more spring discs. It is desirable in an impact absorber for the valve to remain closed until a high pressure has been built up and it is desirable for the flow passage controlled by the valve to have a large cross-section to provide the necessary characteristic. To meet these requirements it is necessary to provide a very strong spring. However, it is also desirable, in an impact absorber, for the valve to open fully with only a small increase in pressure above that needed to open it at all; this implies a relatively low spring rate, which is difficult to achieve in combination with a large initial spring force in a compact arrangement.